THE SANDY CREEPLET. 299 



and compared them, "without any other information, nothing 

 would be more manifest than that we must assign them not 

 only to distinct species, but even to distinct genera. Mr. 

 Alder has favoured me with many specimens, obtained by 

 Mr. Barlee, at Shetland, some of which, each consisting of 

 several full-grown polypes, are perfectly independent and 

 compact, showing not the slightest trace of adhesion to any 

 foreign body, nor of any part that can be distinguished as 

 a root-band. Thus, in the specimen figured in Plate ix. 

 fig. 9, three polypes diverge from a common centre ; others 

 are similarly formed, sometimes with a triangular dilatation 

 of the point of divergence, which thus becomes flat, but 

 still with both surfaces equally entire. I have not seen 

 more than three polypes on any free specimen. 



But among these, we see specimens at first sight hardly 

 distinguishable from them, except by a slight globosity at 

 the point of divergence : when we turn these over, we dis- 

 cover that the globosity has been moulded on a minute 

 shell, evidently that of a Xatica. Then others occur, in 

 which the shell, almost always a Xatica, is larger, and 

 there is a distinct basal carpet uniformly spread over it, 

 of the sand-covered flesh, from which spring four or more 

 polypes : these are manifestly identical with the free ones. 

 But on larger shells the colony of polypes is made up of 

 more individuals ; in one specimen before me, in which the 

 shell is about the size of Xatica Alderi, there are nineteen 

 polypes. In every case the basal carpet has spread in 

 uniform thickness over the entire shell, following the form 

 accurately, and extending to the edge of the outer lip, and 

 clothing the rotundity of the inner lip as far as the eye can 

 follow it. Strange to say, in every example, the shell itself 

 has wholly disappeared, and all that is left is the exact 

 model of it in the sand-clothed membrane, or basal carpet, 

 of the polype. 



